LTE uses OFDMA in downlink and SCFDMA in uplink. The inherent orthogonality of these transmission schemes reduces the intracell interference. The problem of intercell interference impacts the UEs at cell edges because as the frequency is reused across the cells, the edge UEs may be allocated the same subcarriers. As these UEs operate on high power to reach the eNBs, the signals interfere strongly. The edge UEs receive equally strong/weak signals from the adjacent cells, and so the strong interference causes difficulty for the UE when receiving downlink transmission.
Interference is caused because cells only know what radio resources their own UEs are using, and not what other UEs in the neighbor cells are using. For example, in the figure above, Cell A knows what resources A1 is using, but not about what B1 is using, and vice versa. And the cells independently schedule radio resources for their own UEs. So, to the UEs at cell edges (A1 in Cell A and B1 in Cell B), same frequency resource can be allocated.
As seen in the figure below, if the two UEs are located in cell centers like A2 and B2, no interference is caused because they use low power to communicate. However, if they are at cell edges like A1 and B1, their signals cause interference for each other because the two use high power to communicate.
ICIC reduces inter-cell interference by having UEs, at the same cell edge but belonging to different cells, use different frequency resources. Base stations that support this feature can generate interference information for each frequency resource (RB), and exchange the information with neighbor base stations through X2 messages. Then, from the messages, the neighbor stations can learn the interference status of their neighbors, and allocate radio resources (frequency, Tx power, etc.) to their UEs in a way that would avoid inter-cell interference.
For instance, let's say a UE belonging to Cell A is using high Tx power on frequency resouce (f3) at the cell edge. With ICIC, Cell B then allocates a different frequency resource (f2) to its UE at the cell edge, and f3 to its other UE at the cell center, having the one at the center use low Tx power in communicating.
Inter Macro Cell Interference
The interference problem gets even more complicated in small cells, wherein the small cell overlaps the macro cell. As the small cell eNBs operate at a lower power level, the problem extends to ensuring that a UE actually attaches to small cell eNB in proximity, rather that attaching to a macro eNB operating at high power. In this case, the cell edge is the point where the UE receives similar strength signals from both the macro and small cell.
Inter Macro-Small Cell Interference
Smallcell-eNBs have much lower transmit power than macro eNBs. In conventional cellular systems a UE connects to the BS providing the best downlink Signal to Interference-plus-Noise Ratio (SINR), and since the eNB has low transmit power the coverage of small cells will be small. But UEs receiving signals with larger SINR from a macro eNB might have much lower path loss to a small cell eNB and hence cause significant uplink interference to the small cell eNB.
If not placed specifically in a hot-spot, only a small number of UEs will be connected to the small cell-eNB which will limit the gain from offloading the traffic from the macro cells. One way to improve this is to expand the small-cell, for example, by introducing a bias in the cell selection based on reference signal received power (RSRP) or perform UE association determined by minimal path loss. This is often referred to as Cell Range Extention (CRE). In this case UEs at the cell-edge of the pico cell will also experience severe downlink interference from the macro eNB.
Macro-Femto Cell Interference
HeNBs or femtocells use low transmission power, usually lower than the power transmitted by a user terminal. The deployment of HeNBs is usually uncoordinated as opposed to deployment of macro and pico cells which are usually planned. A HeNB is typically operated in closed mode meaning that only certain users that are part of a closeds group (CSG) are allowed to connect to the HeNB.
The CSG restriction introduces complex high-interference scenarios in both the uplink and downlink direction. Consider a UE located close to the HeNB, possibly even in the same room as the HeNB, that are not part of the HeNB’s CSG. In this case the UE will experience severe downlink interference from the HeNB. In fact, the HeNB will often create a coverage hole in the macro cell. Also, the HeNB will experience severe uplink interference from the UE.
Interference is caused because cells only know what radio resources their own UEs are using, and not what other UEs in the neighbor cells are using. For example, in the figure above, Cell A knows what resources A1 is using, but not about what B1 is using, and vice versa. And the cells independently schedule radio resources for their own UEs. So, to the UEs at cell edges (A1 in Cell A and B1 in Cell B), same frequency resource can be allocated.
As seen in the figure below, if the two UEs are located in cell centers like A2 and B2, no interference is caused because they use low power to communicate. However, if they are at cell edges like A1 and B1, their signals cause interference for each other because the two use high power to communicate.
ICIC reduces inter-cell interference by having UEs, at the same cell edge but belonging to different cells, use different frequency resources. Base stations that support this feature can generate interference information for each frequency resource (RB), and exchange the information with neighbor base stations through X2 messages. Then, from the messages, the neighbor stations can learn the interference status of their neighbors, and allocate radio resources (frequency, Tx power, etc.) to their UEs in a way that would avoid inter-cell interference.
For instance, let's say a UE belonging to Cell A is using high Tx power on frequency resouce (f3) at the cell edge. With ICIC, Cell B then allocates a different frequency resource (f2) to its UE at the cell edge, and f3 to its other UE at the cell center, having the one at the center use low Tx power in communicating.
Inter Macro Cell Interference
The interference problem gets even more complicated in small cells, wherein the small cell overlaps the macro cell. As the small cell eNBs operate at a lower power level, the problem extends to ensuring that a UE actually attaches to small cell eNB in proximity, rather that attaching to a macro eNB operating at high power. In this case, the cell edge is the point where the UE receives similar strength signals from both the macro and small cell.
Inter Macro-Small Cell Interference
Smallcell-eNBs have much lower transmit power than macro eNBs. In conventional cellular systems a UE connects to the BS providing the best downlink Signal to Interference-plus-Noise Ratio (SINR), and since the eNB has low transmit power the coverage of small cells will be small. But UEs receiving signals with larger SINR from a macro eNB might have much lower path loss to a small cell eNB and hence cause significant uplink interference to the small cell eNB.
If not placed specifically in a hot-spot, only a small number of UEs will be connected to the small cell-eNB which will limit the gain from offloading the traffic from the macro cells. One way to improve this is to expand the small-cell, for example, by introducing a bias in the cell selection based on reference signal received power (RSRP) or perform UE association determined by minimal path loss. This is often referred to as Cell Range Extention (CRE). In this case UEs at the cell-edge of the pico cell will also experience severe downlink interference from the macro eNB.
Macro-Femto Cell Interference
HeNBs or femtocells use low transmission power, usually lower than the power transmitted by a user terminal. The deployment of HeNBs is usually uncoordinated as opposed to deployment of macro and pico cells which are usually planned. A HeNB is typically operated in closed mode meaning that only certain users that are part of a closeds group (CSG) are allowed to connect to the HeNB.
The CSG restriction introduces complex high-interference scenarios in both the uplink and downlink direction. Consider a UE located close to the HeNB, possibly even in the same room as the HeNB, that are not part of the HeNB’s CSG. In this case the UE will experience severe downlink interference from the HeNB. In fact, the HeNB will often create a coverage hole in the macro cell. Also, the HeNB will experience severe uplink interference from the UE.
